The present invention relates to a method of, and system for, transmitting messages and also to a primary station and a secondary station for use in the system. An example of such a system is an answer back paging system and for convenience reference will be made to an answer back paging system but it is to be understood that the teachings of the present invention can be applied to other 2-way messaging systems.
Answer back paging systems have been disclosed for example in PCT Patent Specification WO96/14716, which corresponds to U.S. Pat. No. 5,872,522. In an elementary form such a system requires a paging network controller (PNC) to arrange for a message to be transmitted to a predetermined addressee. The addressee on receiving a call is able to transmit a simple reply by way of a low power transmitter incorporated into the pager. The above mentioned Patent Specification discloses a system in which a series of messages are transmitted to respective addressees and the PNC then transmits invitations for the addressees to transmit their replies substantially simultaneously as spread spectrum signals which are de-spread at the PNC and the replies forwarded to the respective party requiring the reply. In order to avoid having to apply power control techniques in the pagers to ensure that replies are received by the PNC at comparable power levels, the PNC transmits its invitations at stepwise increasing (or decreasing) power levels and only those pagers just able to receive a respective one of the invitations transmit their replies. A feature of this known method is that relatively strict control of the received power is necessary.
An object of the present invention is to introduce more flexibility into the operation of answer back messaging systems.
According to one aspect of the present invention there is provided a method of operating a message transmission system comprising at least one primary station making transmissions on a down-link and a plurality of secondary stations making transmissions on an up-link, characterised by the primary station transmitting a control signal on the down-link inviting secondary stations wishing to make an up-link transmission to proceed, those of said secondary stations wishing to respond to said invitation making their transmissions, the primary station receiving at least some of the transmissions and analysing those of the received transmissions which are intelligible and determining in the case of those of the analysed, intelligible transmissions which of them comprise responses to messages sent previously on the down-link, acknowledging at least the responses and repeating the transmission of the control signal on the down-link.
The transmissions on the up-link may comprise responses to messages sent on the down-link and/or requests for a service. These transmissions may be in the form of CDMA signals transmitted substantially simultaneously.
According to a second aspect of the present invention there is provided a method of operating a message transmission system, comprising a primary station transmitting messages to a plurality of secondary stations on a down-link, the primary station inviting the secondary stations to transmit their responses on an up-link, the primary station analysing the responses received from the up-link and acknowledging receipt of those responses which were analysed successfully, the primary station determining which messages have been responded to and repeating the invitation to those secondary stations whose responses have not been acknowledged to transmit/re-transmit their responses on the up-link.
According to a third aspect of the present invention there is provided a message transmission system comprising at least one primary station and a plurality of secondary stations, the primary station having means for compiling messages to be sent on a down-link to the secondary stations, means for transmitting a signal on the down-link inviting the secondary stations to transmit responses to the messages on an up-link, means for receiving and analysing the responses, means for acknowledging receipt of responses which were analysed successfully, means for determining which of the transmitted messages have been responded to and repeating the invitation to the secondary stations whose responses have not been acknowledged to transmit/re-transmit their responses, and each of the secondary stations having means for receiving transmissions on the down-link, means for determining if any one of the messages is addressed to it, means for receiving the invitation message from the down-link, means for transmitting a response to a message addressed to the secondary station on an up-link, and means responsive to not receiving an acknowledgement of its response for repeating the transmission of the response when invited to do so.
By means of the method of, and system for, transmitting messages in accordance with the present invention it has been found that the cycle of analysing responses, acknowledging and re-inviting secondary stations not receiving acknowledgements to send or re-send their responses or other up-link transmissions increases the rate of processing of the responses compared to known systems which in turn means that the capacity of the system is increased without the need to increase the amount of infrastructure equipment.
The secondary stations may transmit their responses or other up-link transmissions substantially simultaneously as CDMA signals in which case the stronger response signals will mask the weaker signals. This is known as the near-far problem. In the method in accordance with the present invention the stronger signals on the up-link are detected first and acknowledged and then the secondary stations whose responses or other up-link transmissions have been masked are invited to send or re-send their response signals or other up-link transmissions. After several iterations of the cycle of analysing the responses or other up-link transmissions, acknowledging the successful responses or other up-link transmissions and issuing another invitation or control signal, it has been found that a very high percentage of the responses or other up-link transmissions have been processed. Those which are not processed typically are too weak to be capable of processing and/or the secondary station is either switched-off or out of range.
The time allocated to the above-mentioned cycle may be variable in that a shorter time is allocated to processing a relatively small number of strong responses or other up-link transmissions and a longer time is allocated to processing a relatively large number of weak responses or up-link transmissions and if desired the allocation of time can be made adaptive to the numbers of the response or other up-link transmission signals. This will save processing time compared to the option of allocating intervals of constant duration to processing each batch of responses or other up-link transmissions.
The acknowledgement signal and the invitation signal may be transmitted simultaneously. One method of doing this is for the primary station to transmit messages in an ordered sequence and for the next following invitation message to contain a field comprising elements allocated in an ordered sequence corresponding to the message sequence and in which the elements are adapted to distinguish those responses successfully analysed from those which are unsuccessful, for example by changing a binary xe2x80x980xe2x80x99 to a xe2x80x981xe2x80x99 when successful.
In order to increase message throughput, the population of the secondary stations may be sub-divided into at least 2 groups and while responses or other up-link transmissions from one group ire being analysed the otherwise idle transmitter can be transmitting messages to the other group, and vice versa. The division of the population may be done on the basis of odd numbered and even numbered addresses or on signal strengths (RSSIs).
According to a fourth aspect of the present invention there is provided a primary station for use in a message transmission system in which messages are transmitted on a down-link to a plurality of secondary stations which are capable of transmitting responses on an up-link, the primary station comprising means for compiling messages to be sent on the down-link to the secondary stations, means for transmitting a signal on the down-link inviting the secondary stations to transmit responses to the messages on the up-link, means for receiving and analysing the responses, means for acknowledging receipt of responses which were analysed successfully, means for determining which of the transmitted messages have been responded to and for repeating the invitation to the secondary stations whose responses have not been acknowledged to transmit/re-transmit their responses.
According to a fifth aspect of the present invention there is provided a secondary station for use in a message transmission system in which messages are transmitted on a down-link to a plurality of secondary stations which are capable of transmitting responses on an up-link, the secondary station comprising means for receiving transmissions on the down-link, means for determining if any one of the messages is addressed to it, means for receiving an invitation to reply message from the down-link, means for transmitting a response to a message on the up-link, and means responsive to not receiving an acknowledgement of its response for repeating the transmission of the response when invited to do so.